In recent years, much effort has been put into the preparation of conductive polymers. In WO-A 2004/072159 a polymer latex-based process is disclosed for the preparation of a conductive composition or polymer nano-composite using carbon nanotubes (CNTs) as filler. The carbon nano-tubes are exfoliated and stabilized by the use of water soluble surfactant, for example sodium dodecyl sulphate (SDS), forming a stable homogeneous dispersion, which is further mixed with a polymer latex followed by water removal and melt processing. A homogeneous dispersion of the CNTs in a polymer matrix, formed from the former latex particles, results in an improved stiffness, strength and conductivity of the final polymer nano-composites. The process provides carbon nanotubes reinforced polymers having a percolation threshold at significantly lower loading of the CNTs compared to other mixing techniques. With percolation threshold is meant the amount of CNT needed to retrieve a significant increase in the electrical conductivity of the polymer nano-composites.
The water soluble surfactant, for example SDS, which is often used in practice to stabilize the aqueous dispersion of the exfoliated CNTs, performs excellently as a stabilizing additive but lacks thermal stability which leads to problems during processing of the conductive polymer composition at elevated temperatures. Moreover SDS is a plasticizer for several polymers and therefore lowers the glass transition temperature of the final polymer composite.
It is also known in the art to prepare a conductive coating from a conductive polymer composition by mixing a latex of a conductive polymer (like for example Baytron® or sulphonated polystyrene-stabilised (poly(3,4-ethylenedioxythiophene)) with a non-conductive polymer. The required amount of Baytron in a typical latex composition which is used to make a conductive coating exceeds 24 wt %. These high amounts of conductive polymer make the coating very expensive, sensitive to degradation and the mechanical properties of the coating cannot easily be modified.
There is thus a continuous need to improve the conductivity of polymers or to lower the percolation threshold of polymer nano-composites without being confronted with the above disadvantages.
The aim of the invention is to provide a process for the preparation of a conductive polymer composition or polymer nano-composite which shows a low percolation threshold and/or a high conductivity. It is a further aim of the present invention to provide a conductive polymer composition or polymer nano-composite with improved thermal stability.
This aim is achieved in that the process of the present invention comprises the steps of    A) Providing a latex containing a conductive polymer (A);    B) mixing the latex from A with an aqueous latex of a polymer (B) or with (a) water-soluble precursor(s) of a polymer;    C) removing water from the so obtained mixture;    D) heating the product from step C) to a temperature at which the polymer (B) flows or where the polymer introduced in step B is formed from out of its precursor(s); and    E) processing and/or solidifying the product of step D) into a desired form, wherein the amount of conductive polymer (A) is between 0.1 and 10 wt % relative to the total of all components in step A and B, except for water.